Networks, and in particular mobile networks, are looking for significant improvements in terms of capacity, reliability, consumption, etc. The transmission channel of a mobile network has the reputation of being difficult, and leads to transmission of reliability that is relatively poor. Over the last few years, considerable progress has been achieved in terms of encoding and modulation, in particular concerning consumption and capacity. Specifically, in a mobile network where a plurality of transmitters/receivers share the same resources (time, frequency, and space) it is necessary to reduce transmission power as much as possible.
Such reduction goes against the coverage and thus against the capacity of the system, and more generally against its performance.
In order to increase coverage, make communication more reliable, and more generally improve performance, one approach consists in relying on relays for increasing spectrum efficiency and thus improving the transmission efficiency and the reliability of systems. The topology of MARC systems, as shown in FIG. 1, is such that the resources, nodes S1 and S2, broadcast their encoded information sequences for the attention of the relay R and of the destination D. The relay decodes the signals received from the sources S1 and S2 and it re-encodes them jointly while adding its own redundancy so as to create a spatially-distributed network code. At the destination D, the decoding of the three spatially-distributed encoded sequences, comprising the two encoded sequences received directly from the sources S1 and S2 and the encoded sequence coming from the relay, relies on joint channel/network decoding algorithms.
Network coding is a form of cooperation in which the nodes of the network share not only their own resources (power, bandwidth, etc.) but also their computation capacity, in order to create a distributed code that becomes more and more powerful as the information propagates through the nodes. It gives rise to substantial improvement in terms of diversity and of encoding, and thus in terms of transmission reliability.
For a MARC system, it is considered that the data rate in bits per second (bits/s) of the sources and of the relay is 1/Ts and that the total available transmission duration is set at T. Thus, the number of available channel uses that can be shared between the sources and the relay is N=DT. If consideration is given to using the Nyquist data rate and transmitting a pulse of sinc (cardinal sine) waveform, then N is the total number of available complex dimensions and D is the total available bandwidth of the system.
A distinction is made between two types of operation for the relay: half-duplex mode and full-duplex mode.
In the known half-duplex mode, a distinction is drawn between two stages of transmission that correspond to different transmission channel uses, since the relay is not capable of receiving and transmitting simultaneously. The sources and the relay thus split the total number of transmission channel uses in two, corresponding to the two stages. During the first stage, which comprises the first transmission channel uses (time slots), both sources transmit, but not the relay. The relay performs joint decoding/re-encoding in order to deduce the signal for transmission during subsequent transmission channel uses. During the second stage, which comprises the second transmission channel uses, the relay transmits the signal as determined during the first transmission channel uses, and the sources transmit the second parity sequences relating to the same information as was transmitted during the first transmission channel uses. The relay thus complies with certain timing that is set by the duration of the second stage. Half-duplex type relays are attractive because they present a simple communications scheme and because they are easy to implement and thus of low cost.
PCT patent application WO2012/022905 A1 in the name of the same applicant describes a half-duplex relay that operates using the above two-stage timing for a MARC system having non-orthogonal links. The relay performs a relaying method in which words that are decoded erroneously are not transmitted, in application of a technique known as selective decode and forward (SDF). In that technique, the relay attempts to decode the messages from the sources and transmits the result of a deterministic function of only those messages that are detected without error. The error detection is performed using a cyclic redundancy check (CRC) that is included in the source messages.
Although the selective relaying technique presents undeniable advantages by avoiding error propagation by the relay, its use with a half-duplex relay has the drawback of requiring that the relay and the sources determine and know the respective durations of the two transmission stages.